11.2:
Meiosis I
In humans, sex precursor cells undergo meiosis, a process with two divisions named meiosis one and two, to generate haploid sperm or egg cells.
Prophase one commences meiosis one, where a diploid cell's chromatin, involving one paternal and one maternal genetic set, condenses. This forms typical x-shaped chromosomes, which anchor to the nuclear envelope.
Father and mother inherited copies of the same chromosome are joined by the development of protein threads between them and oriented, so that equivalent genes line up.
Such homologous chromosomes then swap pieces during crossing over and remain affixed at the spot of exchange even as the threads between them dissolve. Outside the nucleus the meiotic spindle apparatus appears, which involves microtubules emanating from centrosomes.
During prometaphase one the nuclear envelope disperses, and protein kinetochores form on centromeres. Microtubules then elongate and connect to these structures, fastening each homologous chromosome in a pair to a different pole.
With metaphase one the pairs position randomly along the cell's midline and anaphase one is marked by the retraction of mictrotubules, which splits homologous chromosomes.
The cell also lengthens and telophase one follows, where chromosomes settle at opposite sides, slacking and are encircled by nuclear envelopes. Concurrently the cytoplasm splits, forming a cell pair.
Thus meiosis one ends with two genetically distinct haploid cells, that each contain one chromosome from every homologous pair initially present.
11.2:
Meiosis I
Meiosis is a carefully orchestrated set of cell divisions, the goal of which—in humans—is to produce haploid sperm or eggs, each containing half the number of chromosomes present in somatic cells elsewhere in the body. Meiosis I is the first such division, and involves several key steps, among them: condensation of replicated chromosomes in diploid cells; the pairing of homologous chromosomes and their exchange of information; and finally, the separation of homologous chromosomes by a microtubule-based network. This last step segregates homologs between two haploid precursor cells that may subsequently enter the second phase of meiosis, meiosis II.
Crossing Over and the Synaptonemal Complex
The exchange of equivalent segments between homologous chromosomes occurs early on during meiosis I, and is referred to as crossing over. This process relies on the close association of such homologs, which are drawn together by the formation of a connective protein framework called the synaptonemal complex between them. To function correctly, the complex requires three parts: (1) vertical lateral elements, which form along the inward-facing sides of two juxtaposed homologous chromosomes; (2) a vertical central element positioned between the chromosomes; and (3) transverse filaments, or horizontal protein threads that connect the vertical and central components. The result has often been compared to a ladder, with the lateral elements serving as the legs and the transverse filaments akin to rungs. Importantly, the synaptonemal complex helps to precisely align homologous chromosomes, enabling crossing over between equivalent stretches of genetic material; however, this framework is transient, with most of it dissolving after such recombination occurs.
Meiosis and Chromosomal Abnormalities
Meiosis is a complicated process, and errors can happen despite cellular safeguards. Occasionally, such mistakes are the result of nondisjunction, where chromosomes are not evenly partitioned between cells. During meiosis I, this means that a pair of homologous chromosomes may end up in one of the two resulting cells, while the other lacks the chromosome altogether. When the precursor that received both homologs enters and completes meiosis II, both daughter cells formed possess two copies of the chromosome in question, rather than the single copy expected.
One of the more well-known results of nondisjunction occurring during meiosis I is trisomy 21, in which an individual has three copies of chromosome 21. Commonly known as Down syndrome, this condition is characterized by distinct facial features, developmental delays, and heart defects. Although the exact cause of nondisjunction resulting in Down syndrome and other trisomies is variable, it may be the result of problems with the microtubule apparatus that separates the chromosomes, or defects in proteins that join chromosomes together.
Suggested Reading
Kazemi, Mohammad, Mansoor Salehi, and Majid Kheirollahi. “Down Syndrome: Current Status, Challenges and Future Perspectives.” International Journal of Molecular and Cellular Medicine 5, no. 3 (2016): 125–33. [Source]